The ν5 band of CH3CD3: high-resolution spectrum and global three-band analysis

The lowest frequency parallel fundamental band ν5 of CH3CD3 near 900 cm−1 has been measured at low temperature with a resolution of 0.002 cm−1 using Fourier transform spectroscopy. The band is weak and an absorption path of 60 m was used. Large torsional splittings due to inter-vibrational coupling have been observed. Building on previous studies of the torsional levels in the ground vibrational state and in the methyl rocking state ν12, a three-band analysis including these most recent data have been completed. The combined data set of almost 2 700 frequencies was fitted to within experimental accuracy using a 45-term model Hamiltonian. The results were found to bear a striking resemblance in most respects to those of an earlier, analogous study of CH3SiH3. In addition to the interactions described in that work, however, the present study indicates an important but local coupling between the (v5=1,v6=0) and (v12=1,v6=1) states of CH3CD3. This coupling has a strong effect on the K=7 lines of the ν5 band, which have been successfully assigned and fitted here.